The present invention relates to signal reproducing methods and signal reproducing apparatuses, and more particularly to a method and apparatus for reproducing a signal which is recorded with a high recording density.
Due to the rapid information transfer capabilities and large storage capacity of recent magnetic recording and reproducing devices, there is a demand to improve the recording density of magnetic recording and reproducing apparatuses. Various systems using waveform interference have been proposed to improve the recording density.
In prior art magnetic recording and reproducing apparatus, a waveform interference technique called Partial Response Maximum Likelihood (PRML) technique is employed to improve recording density. The PRML technique is a signal processing technique for realizing a high-density recording on magnetic disk and optical disk, and combines the Partial Response (PR) technique and the Maximum Likelihood (ML) technique.
FIG. 8 is a diagram showing the general construction of a magnetic disk unit 61. The magnetic disk unit 61 includes a magnetic disk 62 that is rotated by a spindle motor 63, and a magnetic head 64 which faces the magnetic disk 62. A recording signal is recorded on the magnetic disk 62 by generating a magnetic field dependent on the recording signal of the magnetic head 64. The recording signal recorded on the magnetic disk 62 is later reproduced by the magnetic head 64, which detects changes in the recorded magnetic poles.
The magnetic head 64 is connected to a voice coil motor (which is not shown) via an arm 65, and is moved in a radial direction on the magnetic disk 62, as indicated by arrow A. Signals are recorded on tracks of the magnetic disk 62, and the tracks are formed about a rotation center of the magnetic disk 62.
The magnetic head 64 is coupled to a recording system 66, which processes the recording signal, and a reproducing system 67, which reproduces a head reproduced signal that is reproduced by the magnetic head 64. The recording system 66 includes an encoder 68 which encodes the recording signal received from an input terminal T.sub.IN, and a recording equalizer 69, which equalizes the encoded recording signal from the encoder 68 so that a high-density recording is possible. The output signal of the recording equalizer 69 is supplied to the magnetic head 64.
The reproducing system 67 includes a reproducing equalizer 70, an ML detector 71, and decoder 72. The equalizer 70 receives the high-density head reproduced signal from the magnetic head 64, which converts the magnetic information into electrical information. The equalizer 70 equalizes the head reproduced signal using waveform interference. The ML detector 71 reproduces an output signal from the equalizer 70 into the original signal using waveform interference. The decoder 72 reproduces the recording signal from the output signal of the ML detector 71, and outputs the recording signal through an output terminal T.sub.OUT.
The magnetic disk unit 61 performs signal processing using waveform interference for high-density recording through the use of the recording equalizer 69, the reproducing equalizer 70, the ML detector 71 and the other components discussed. Techniques that have been proposed for carrying out such signal processing are the PR technique and a Fixed Delay Tree Search with Decision Feedback (FDTS/DF) technique.
When attempts are made to further improve the high-density recording by using the PR technique, the FDTS technique and the like, it becomes essential to reduce the equalization loss and the correlation noise in the reproducing equalizer 70, the ML detector 71 and the other components found in the reproducing system 67.
FIG. 9 is a block diagram showing the construction of a reproducing system employing the PRML technique. As shown in FIG. 9, the head reproduced signal is received through an input terminal IN and is supplied to a feedforward filter 81, which corresponds to the reproducing equalizer 70 shown in FIG. 8. The feedforward filter 81 carries out a wave-shaping process with respect to the head reproduced signal so that the level of the head reproduced signal will not affect the preceding signal. The feedforward filter 81 also equalizes the target waveform by controlling the waveform interference constant to the subsequent signal. The signal which is equalized to the target waveform by the feedforward filter 81 is supplied to an ML detector 82, which corresponds to the ML detector 71 shown in FIG. 8. The ML detector 82 detects the most likely sequence of the signal from the feedforward filter 81 having the target waveform and makes a correlation from the waveform interference. An output signal of this ML detector 82 is output through an output terminal OUT.
In the reproducing system employing the PRML technique, the equalization loss in the feedforward filter increases as the line density increases. In order to reduce this equalization loss, an impulse response such as PR4, EPR4 or EEPR4 is used in the feedforward filter 81.
FIG. 10 shows impulse response characteristics of the various impulse responses. FIG. 10 (A) shows the impulse response characteristic of the PR4, and FIG. 10 (B) shows the impulse response of the EPR4, and FIG. 10(C) shows the impulse response of the EEPR4.
The PR4 is sometimes also referred to as PR (1, 0, -1), and has the impulse response shown in FIG. 10 (A). This impulse response is realized by giving to the reproduced signal a characteristic (1-D) (1+D), where D denotes a 1-bit delay.
The EPR4 is sometimes also referred to as PR (1, 1, -1, -1), and has the impulse response shown in FIG. 10 (B). This impulse response is realized by giving to the reproduced signal a characteristic (1-D) (1+D).sup.2, where D denotes a 1-bit delay.
The EEPR4 is sometimes also referred to as PR (1, 2, -2, -1), and has the impulse response shown in FIG. 10 (C). This impulse response is realized by giving to the reproduced signal a characteristic (1-D) (1+D).sup.3, where D denotes a 1-bit delay.
However, even when the impulse responses such as the PR4, EPR4 or EEPR4 are used, while it is possible to reduce the increase of the equalization loss, the equalization loss caused by the increase of the line density still cannot be prevented.
FIG. 11 shows the equalization loss with respect to a normalized line density of the recording signal for the PR4, EPR4 and EEPR4. In FIG. 11, the equalization loss of the PR4 is indicated by a solid line, the equalization loss of the EPR4 is indicated by a dotted line, and the equalization loss of the EEPR4 is indicated by a one-dot chain line.
As shown in FIG. 11, the equalization loss of the EPR4 is smaller than that of the PR4, and the equalization loss the EEPR4 is smaller than that of the EPR4. However, the equalization loss increases as the line density increases, for each of the PR4, EPR4 and EEPR4.
FIG. 12 shows equalization gains with respect to the normalized frequency for the PR4, EPR4 and EEPR4. In FIG. 12, the equalization gain of the PR4 is indicated by a solid line, the equalization gain of the EPR4 is indicated by a dotted line, and the equalization gain of the EEPR4 is indicated by a one-dot chain line.
As shown in FIG. 12, the noise increases because the equalization gain greatly changes depending on the normalized frequency. For this reason, the correlation noise at the ML detector 82 cannot be neglected.
FIG. 13 shows signal-to-noise (S/N) ratios with respect to the normalized line density for the PR4, EPR4 and EEPR4. In FIG. 13, the S/N ratio of the PR4 is indicated by a solid line, the S/N ratio of the EPR4 is indicated by a dotted line, and the S/N ratio of the EEPR4 is indicated by a one-dot chain line. As shown in FIG. 13, the S/N ratio becomes smaller as the line density increases, and the effects of the S/N ratio are more notable as the line density increases.
The convolution steps increase in the EPR4 as compared to the PR4, and the convolution steps increase in the EEPR4 as compared to the EPR4. As the convolution steps increase, it is possible to reduce the noise at the ML detector 82.
FIG. 14 shows S/N ratios at a portion of the ML detector 82 with respect to the normalized line density for the PR4, EPR4 and EEPR4. In FIG. 14, the S/N ratio of the PR4 is indicated by a solid line, the S/N ratio of the EPR4 is indicated by a dotted line, and the S/N ratio of the EEPR4 is indicated by a one-dot chain line. As shown in FIG. 14, the S/N ratio of the ML detector 82 is larger for the EPR4 and the EEPR4 than for the PR4.
FIG. 15 shows peak signal/root mean square (RMS) noise with respect to the normalized line density for the PR4, EPR4 and EEPR4. In FIG. 15, the peak signal/RMS noise of the PR4 is indicated by a solid line, the peak signal/RMS noise of the EPR4 is indicated by a dotted line, and the peak signal/RMS noise of the EEPR4 is indicated by a one-dot chain line.
As shown in FIG. 15, the noise is reduced more for the EPR4 than for the PR4, where the EPR4 has more convolution steps compared to the PR4. The noise is reduced more for the EEPR4 than for the EPR4, where the EEPR4 has more convolution steps compared to the EPR4. Accordingly, the noise can further be reduced by increasing the convolution steps, but when the convolution steps are increased, the number of registers required in the ML detector 82 also increases.
When determining the number of registers required, the number of convolution steps may be denoted by T, paths 0 and 1 are provided for each step, and by taking into consideration the combination of such paths, it becomes necessary to provide 2.sup.(T+1) paths. The number of registers required in 1 path is normally 5.multidot.(T+1), .multidot.2.sup.(T+1) and thus the number R of registers required for the paths can be described by the following formula (1). EQU R=5.multidot.(T+1).multidot.2.sup.(T+1) (1)
For PR4, the number T of convolution steps is "1". Thus, from the formula (1) above, the number R of registers required for the PR4 becomes: EQU R=5.multidot.(1+1).multidot.2.sup.1+1 =5.multidot.2.multidot.2.sup.2 =40.
According to the EPR4, the number T of convolution steps is "2", and from the formula (1) the number R of registers required from the EPR4 becomes 120. According to the EEPR4, the number T of convolution steps is "3", and from the formula (1), the number R of registers required for the EEPR4 becomes 320. In other words, the number R of registers that are required increases exponentially as the number T of convolution steps increase.
Therefore, when using the PR technique, the number of convolution steps becomes small if a small circuit scale is used, thereby increasing the noise and more easily generating errors. On the other hand, the circuit scale will become extremely large if the noise is to be reduced.
The FDTS/DF technique has been proposed to eliminate the above problems of the PR technique (the increased equalization loss, the increased correlation noise and the considerably increased circuit scale).
FIG. 16 is a block diagram showing construction of the reproducing system employing the FDTS/DF technique.
The head reproduced signal is supplied to a feedforward filter 91 from an input terminal IN. The feedforward filter 91 carries out a wave-shaping process with respect to the head reproduced signal so that the level of the head reproduced signal will not affect the preceding signal. The feedforward filter 91 also equalizes the target waveform by controlling the waveform interference to the subsequent signal to be constant. The signal which is equalized to the target waveform by the feedforward filter 91 is supplied to a subtracter 92.
The subtracter 92 subtracts an output signal which is output from an output terminal OUT and is received via a feedback filter 93 from the signal received from the feedforward filter 91. An output signal of the subtracter 92 is supplied to a FDTS processor 94.
The feedback filter 93 multiplies a waveform interference coefficient by the output signal that is output via the output terminal OUT. The feedback filter 93 also obtains a waveform interference quantity of the preceding signal and the waveform interference quantity of the subsequent signal. The output signal of this feedback filter 93 is supplied to the subtracter 92.
The FDTS processor 94 obtains a mean-square error between an anticipated value under a noise-free condition and the signal which is output from the subtracter 92, and outputs a most likely path out of the possible paths which are arranged in a tree format. The output signal of the FDTS processor 94 is output via the output terminal OUT and is also supplied to the feedback filter 93.
FIG. 17 shows an equalization loss versus normalized frequency characteristic of the FDTS/DF feedforward filter. More particularly, FIG. 17 shows the equalization loss with respect to the normalized frequency for a case where an impulse response Y(D) of the feedforward filter 91 is set to Y(D)=1+D+(1/2)D.sup.2 +(1/4)D.sup.3 . . . and a normalized line density K of an input signal is set to 3.0, 2.5 and 2.0. In FIG. 17, the smaller the normalized line density, the smaller the equalization loss. In addition, the equalization loss can be set below 2 dB to a relatively low value at the normalized line density of 2.0, as shown in FIG. 17.
FIG. 18 shows another equalization loss versus normalized frequency characteristic of the FDTS/DF feedforward filter. More particularly, FIG. 18 shows the equalization loss with respect to the normalized frequency for a case where the impulse response Y(D) of the feedforward filter 91 is set to Y(D)=1+1.5D+D.sup.2 +0.5D.sup.3 +(1/4)D.sup.4 . . . and the normalized line density K of the input signal is set to 3.0, 2.5 and 2.0. In FIG. 18, the smaller the normalized line density, the smaller the equalization loss. In addition, the equalization loss can be set below 2 dB to a relatively low value at the normalized line density of 2.0, as shown in FIG. 18.
Accordingly, the equalization loss of the feedforward filter 91 can be set relatively small as shown in FIGS. 17 and 18 by the FDTS/DF, and the equalization loss is small compared to that obtained by the PR4, EPR4 and EEPR4 described in conjunction with FIG. 12.
FIG. 19 shows an equalization loss versus frequency characteristic for the PR4, EPR4, EEPR4 and FDTS/DF. As may be seen from FIG. 19, a characteristic FDTS1 which is obtained by the FDTS/DF using the feedforward filter having the characteristic shown in FIG. 17 has a small equalization loss for all normalized line densities as compared to that obtained using the PR4. In addition, a characteristic FDTS2 which is obtained by the FDTS/DF using the feedforward filter having the characteristic shown in FIG. 18 has a small equalization loss for all normalized line densities as compared to that obtained using any of the PR4, EPR4 and EEPR4, thereby making it possible to reduce the correlation noise compared to the PR4, EPR4 and EEPR4.
According to the conventional reproducing system employing the PR technique, the number of convolution steps is reduced if the circuit is realized on a small circuit scale. However, the use of a small circuit scale also increases the noise and errors are generated more easily. On the other hand, to reduce the noise, the circuit must be realized on an extremely large circuit scale. In this latter case, it would require further size reduction of mechanical parts in order to secure a sufficiently large mounting space for the large scale circuits, but there is a limit to size reduction for the mechanical parts. Accordingly, a rather large mounting space is still required.
In addition, if the path length is short, it is possible to reduce the equalization loss and the correlation noise. However, the short path length will deteriorate the performance of the ML detector, and then the error propagation becomes large.
The object of the present invention is to provide a signal reproducing method and a signal reproducing apparatus which can suppress the equalization loss and the correlation noise, but will not deteriorate the performance of the ML detector.